Effect of Surface Morphology on the Stability of Thin Nanostructures

We have recently studied the atomic scale structural stability of
freestanding wavy gold (Au) nanofilms using molecular dynamics
simulations. In recent years, wavy or patterned structurs have shown great promise for applications in various emerging technologies including fuel cells
engineering, tissue engineering, biomedical engineering, creation
of counterfeit-resistant documents , nanolithography in microelectronics, optoelectronics, nanomachinesand many others. It is out of question that the success of these novel applications lies on one crucial factor – the
integrity and stability of the patterned structures during their manufacturing
process as well as during their service life.
However, predicting material stability from the macroscopic physical
laws often becomes challenging due to the size dependent behavior of
nanostructured materials. The size dependency in nanostructures are primarily arises from "surface effects" and "quantum effects" where surface effects include activities at the free surface due to surface energy and surface stress, and quantum effects include quantum confinement effect or quantum hall effect. Between these two effects, "surface effects" play the primary role in determining the structural stability of materials.

In
this study, we have obtained equilibrium configurations of various wavy gold films at different temperatures using molecular dynamics simulations. The degree of waviness was
controlled by varying the wavelength of a sinusoidal surface. Four different
films were thus constructed. The
stability of these thin films was studied at five different temperatures which
were below the melting point of bulk gold. 
Some critical observations are:

(a) It was observed from the equilibrated shapes of these structures that
the stability of wavy surfaces significantly depends on temperature and degree
of waviness.

(b) It was also found that the size dependent melting of Au films occurred
in a progressive manner over a range of temperature rather than occurring at
some fixed temperature - as expected from fundamental thermodynamics of bulk crystal.

We have proposed a new method to identify the progressive melting temperature of crystal structures and using this method we have demonsrtated that the progressive melting is caused by the non-uniform
melting of film along the thickness direction. The melting in the film starts when
only the atomic layers at the two free surfaces melt at temperatures lower than
the so-called bulk melting temperature. As heating continues, the entire film
then gradually melts.

We have discussed that the low temperature pre-melting of the surface boundary
layers plays the foremost role in the instability of wavy films. It is shown
than for a particular temperature, e.g. at 810 K, there exist a critical film thickness
below which melting occurs in the entire structures; otherwise it is confined
at the boundary layers only.  It is
recognized that the film thickness of the wavy films were five times thicker
than the critical thickness at 810 K. However, it was demonstrated that the surface
waviness in the films behaved as secondary thin films, and for smaller
wavelengths, the major fractions of the sub-film thickness fell below the
critical thickness at 810K. As a result, some wavy films became unstable. At
elevated temperatures, e.g. 1080 K or higher, films are unable to maintain
their structure due to melting point depression of Au films.  

In short, the current work demonstrates that stability of wavy surface are critically dependent on temperature and degree of surface roughness.

This work has been accepted for publication in Nanotechnology and will appear online on June 2008.